122 THE THIRD DAY. [CHAP. 



the third day marked off from the rest by a slight 

 constriction. This distinction, which becomes much 

 more evident later on by a thickening of the walls and 

 roof of the front portion, separates the hind-brain into 

 the cerehellum in front, and the medulla oblongata 

 behind (Figs. 38 and 39). While the walls of the 

 cerebellar portion of the hind-brain become very much 

 thickened as well at the roof as at the floor and sides, 

 the roof of the posterior or medulla oblongata portion 

 thins out into a mere membrane, forming a delicate 

 covering to the cavity of the vesicle (Fig. 40, iv), which 

 here becoming broad and shallow with greatly thick- 

 ened floor and sides, is known as the fourth ventricle, 

 subsequently overhung by the largely developed pos- 

 terior portion of the cerebellum. 



The third day, therefore, marks the differentiation 

 of the brain into five distinct parts : the cerebral 

 hemispheres, the central masses round the third 

 ventricle, the corpora bigemina or optic lobes, the 

 cerebellum and the medulla oblongata; the original 

 cavity of the neural canal at the same time passing 

 from its temporary division of three single cavities into 

 the permanent arrangement of a series of connected 

 ventricles, viz. the lateral ventricles, the third ventricle, 

 the iter (with a prolongation into the optic lobe on 

 each side), and the fourth ventricle. 



At the same time that the outward external shape 

 of the brain is thus being moulded, internal changes 

 are taking place in the whole neural canal. These are 

 best seen in sections. 



At its first formation, the section of the cavity of 

 the neural canal is round, or nearly so. 



